The invention relates generally to hydraulic cylinders. More particularly, it relates to welded hydraulic cylinders formed of a cylindrical tube and an end cap or plug that is circumferentially welded to the cylinder to enclose one end of the cylinder. More particularly, the invention relates to methods of reducing hoop stress at the weld joint in the cylinders.
Hydraulic cylinders are used in a wide variety of industrial applications. One of the more common uses is as actuators on work vehicles. Work vehicles, such as agricultural tractors, road graders, telehandlers, skid steer loaders, mobile drilling rigs, use either single or double acting hydraulic cylinders to move various components of the work vehicle and to move implements attached to the work vehicle with respect to the vehicle and with respect to each other.
A common method of manufacturing these cylinders is to machine and polish the inside diameter of a cylindrical tube. A plug or end cap is machined to enclose one end of the tube through which fluid will be introduced or removed from the cylinder. The end plug is partially inserted into the cylindrical tube, is clamped in a rotational welding machine, and is rotated in that machine while a circumferential weld is made that bonds one end of the tube to a portion of the plug.
To ensure that the plug and the cylindrical tube are properly aligned during the welding process, the plug is usually provided with a small pilot portion on one end that is inserted into the tube. This pilot portion has a smaller diameter than the rest of the plug portion and the junction between these two portions, often called the pilot portion and the eye portion, is formed as a planar or conical shoulder. To assemble the tube and plug, the pilot portion is inserted into the tube until the shoulder on the plug abuts an end face of the tube. The weld is formed between the end face of the tube and the abutting shoulder portion of the plug.
A common failure mode for such welded cylinders is that of weld failure. Hydraulic pressure acting against the inside surface of the tube creates hoop stress, which tends to cause the tube to expand, to increase in diameter. The plug, on the other hand, is typically made of a very large, solid piece of steel that does not expand when hydraulic fluid presses against its internal surfaces. As a result, a very high bending stress is created right at the weld joint that couples the tube and the plug. The tube expands radially when pressure is applied. The plug does not expand. Since the junction between the tube and the plug is the circumferential weld joint, it is the circumferential weld joint where the stress is maximum.
One way of avoiding failures at the tube-to-plug joint has been to provide a more flexible coupling. For example, rather than employing a weld to join the tube and plug, many cylinders, especially smaller cylinders, use a thread joint between the tube and plug. In these cylinders, a pilot portion of the outside diameter of the plug is threaded, and a corresponding inside portion of the end of the tube is also threaded. To couple the two together, the threads on the outside of the plug are engaged with the threads on the inside of the tube and the two are threaded together. When hydraulic fluid under pressure is introduced into the cylinder, the tube expands slightly due to the hoop stress generated by the fluid. Since the bond between the tube and the plug is a thread joint, the tube is free to expand slightly thereby slightly increasing the gap between the tube and the plug. This non-restrictive joint allows slight expansion of the tube to occur without additional stresses of a joint trying to restrain it. In this manner, the tube is made stronger. In addition, by eliminating the weld joint, the xe2x80x9ccastxe2x80x9d portion of the cylinder, the cylinder is made much more resistant to stress generally.
Of course, since the tube is permitted to expand with respect to the plug, a gap between the two, along the thread joint is created. This gap, although small, provides a fluid leakage path. Fluid inside the cylinder will leak out of the cylinder along this thread joint. For this reason, a fluid tight seal that is relatively flexible is placed between the plug and the tube. In smaller cylinders, this may be nothing more than a wrapping of thin Teflon(copyright) tape around the external threads on the plug. For larger cylinders, however, such as those that have an area greater than about xc2xdxe2x80x3 in diameter, an O-ring is typically placed in a circumferential groove in the plug before it is inserted into the tube. The O-ring extends circumferentially around the diameter of the plug and abuts both the plug and the tube providing a generally fluid-tight seal between the two that prevents fluid in the cylinder from leaking out between the threads on the plug and the mating threads on the tube. When the tube in these threaded cap arrangements are pressurized with hydraulic fluid, they expand. The O-ring, however, is selected to have a sufficient pre-load to maintain contact with the internal walls of the tube even when it expands slightly due to hoop stress.
What is needed, therefore, is an improved hydraulic cylinder design that provides the low cost and ease of manufacture of a welded cylinder, yet reduces the longitudinal tensile forces on the weld to increase the cylinder""s longevity. It is an object of this invention to provide such a hydraulic cylinder.
In accordance with a first embodiment of the invention a hydraulic cylinder for use in a work vehicle is provided that includes a cylinder portion having a circular cylindrical tube with a first end, a threaded second end, an inner cylindrical surface with an inner diameter and a tube longitudinal axis, a plug having a pilot portion disposed inside the tube and an eye portion extending outside the tube and a plug longitudinal axis, a circular weldment extending around the circumference of the plug between the plug portion and the eye portion and coupling the first end of the tube to the plug between the pilot portion and the plug portion, and a seal disposed between and sealing against the pilot portion and the inner cylindrical surface of the tube. In addition to the cylindrical portion the cylinder includes a threaded end plug configured to be engaged with threads on the threaded second end of the tube to enclose and seal the second end of the tube and defining a central longitudinal rod opening, and a piston assembly including a piston configured to be slidingly supported within the cylindrical tube and a piston rod fixed thereto an extending out of the cylindrical tube through the rod opening.